Right before entering the door to this boss fight, visit the merchant to the left of it (its after beating all those Los Illuminados on the lift). Check the price of the Rocket Launcher...if you don't have enough money to buy it, sell whatever items or treasures you can spare to afford it. Also, make sure that you have both item box extensions purchased and have enough room to fit the rocket launcher into your inventory. After buying it, make sure you save at the typewriter that's also there and go through the door to face Salazar. When the fight begins, make sure you have your shotgun weapon equipped and shoot directly at the large eye of the long necked creature in front of you. Now another cut scene should appear which exposes Salzaar. Quickly switch to the rocket laucher, aim directly for Salazar and fire! This should immediately end the boss fight. If anything goes wrong, reload the game and try again.
Credit to ¦?¦ ¦ ¯¦¯ THAT BUTTON-? for the video